1. Field of the Invention
The present invention relates to novel compositions of matter for foundry molds and cores, and, more especially, to such compositions comprising a silicate based binder.
2. Description of the Prior Art
It has long been known to this art to prepare foundry cores and molds by utilizing a ternary mixture comprising 93 to 98 parts by weight of a sand suitable for foundry purposes, 2 to 7 parts by weight of an aqueous solution of an alkali metal silicate and a stoichiometric amount, relative to the silicate, of an organic carbonate ester. Compare U.S. Pat. No. 3,137,046. Molds having satisfactory mechanical properties are obtained by this method.
Such compositions typically were hardened with carbon dioxide. The process was said to be improved by employing different additives therein, as per French Pat. No. 1,172,636. Finally, an attempt was made to eliminate the use of carbon dioxide gas, especially in the molding of the larger shaped articles, because the gaseous hardening agent displayed certain disadvantages, particularly at elevated temperatures.
Thus, in U.S. Pat. No. 3,203,057, it is proposed to use a binder comprising an aqueous solution of an alkali metal silicate and an additive consisting essentially of 3 to 100% alumina with respect to the binder.
However, it is known that, of these compositions, numerous desirable properties are required. In particular, good mechanical properties are expected of the cores and molds. It is further desired to simultaneously obtain shorter setting times and a relatively extended useful life.
"Useful life" is defined as that period of time during which it is possible to store the mixture of sand, silicate and hardener, without a reduction in mechanical properties, from the moment that the silicate is added to the mixture of sand plus hardener.
"Setting time" is defined as that period of time which intervenes between the moment that the silicate is added to the mixture of sand plus hardener to that point in time when modifications are no longer observed on the surface of the agglomerated sand. In practice, the point of setting is determined simply by exerting pressure on said surface of the sand mix.
The immediately foregoing can be said to characterize those reasons why it was recently proposed, in U.S. Pat. No. 4,213,785, assigned to the assignee hereof, to employ hardening or curing catalysts of aqueous solutions of alkali metal silicates based on alkylene carbonates and additionally containing methyl esters of the organic acids.
But even though such hardeners, used in combination with sand and alkali metal silicate based binders, afford excellent results as regards ultimate mechanical properties, useful lives and setting times, one problem remained without a satisfactory solution, namely, demolding. "Demolding" is defined as that step of extracting or removing with relative ease the sand located within the cavities formed by the cores, from the metal casting.
In effect, the agglomerated sand comprising the various molds and cores must have a high mechanical strength prior to the pouring of the molten metal and must retain a satisfactory mechanical strength at elevated temperatures, but same should have mechanical properties such that the sand may be readily extracted or removed, once the metal has cooled.
To facilitate demolding, it too is known to this art to employ certain carbonaceous materials and/or film-forming resinous adhesives [French Pat. No. 2,237,706], but the phenomenon of recarburization may occur.
And consistent with U.S. Pat. No. 4,233,076, also assigned to the assignee hereof, essential solution to the aforesaid problem is found by utilizing foundry mold and core compositions comprising sand, a binder based upon an alkali metal silicate, and a hardening agent, by including in such compositions a demolding agent which comprises at least in part, an alumina having an average grain size of less than 40.mu., and preferably an average grain size of between 0.2 and 5.mu..
Such compositions afford excellent results, notably in the case of high temperature treatments, e.g., the casting of iron or steel. Nonetheless, the efficacy of the process declines if the nature of the metal being cast mandates lower operating temperatures, e.g., on the order of 750.degree. C.
Also compare French Pat. Nos. 1,545,740 and 2,014,408.